Various methods have been developed to reshape the refractive window of the human eye, the cornea, in order to correct for the typical vision defects. Among these are nearsightedness (myopia), with the unaccommodated nominal focusing plane falling before the retina; farsightedness (hyperopia), with focusing plane beyond the retina and the combination of defects known as astigmatism, in which the cornea has a toroidal shape and there is no plane of best focus. The most common methods for vision correction for these defects are spectacles, and contact lenses (hard, soft and gas permeable types) which provide the correct amount of refractive power to shift the unaccommodated focusing plane to its optimum position on the retina. Glasses and contact lenses, when made to a proper prescription, provide vision correction to an accuracy of about .+-.0.25 diopter and best visual acuity. However, glasses are worn externally and are sometimes perceived to be uncomfortable, inconvenient, or detracting from personal appearance. They may actually impede certain activities such as sports, photography (or other view finder activities), aircraft flying and the like. Spectacles are sometimes misplaced and can be difficult to find if the natural error is large, i.e., the misplacer can't see them to find them. Contact lenses sometimes are utilized where use of glasses has been considered to be undesirable, mostly for cosmetic reasons. Contact lenses however, entail problems of their own in terms of possible eye infection with misuse and the necessity for specialized and time consuming procedures required to maintain sterility and minimize contamination. More importantly, many people cannot tolerate the insertion of foreign objects on or in their eyes. Whereas spectacles can be taken off and put on again as necessary, contacts are much less convenient in this respect. Contact lenses also tend to be expensive compared to spectacles.
In response to a need for safe permanent correction of vision, without recourse to glasses or contact lenses, two major surgical methods of vision correction have evolved. The first, radial keratotomy (RK), involves surgical incision of the cornea, with deep radial cuts outside the vision zone which cause a roughly predictable flattening of the cornea and a reduction in refractive power thereof, suitable for correcting low levels of myopia. This is however, a major surgical procedure requiring considerable skill in order to achieve the desired refractive correction. Though undercorrection errors are correctable, overcorrections are not. Additionally, the refraction unpredictably progresses toward hyperopia over long periods of time, i.e., about ten years.
The second procedure is photo-chemical and thermal corneal ablation with an excimer laser (photo refractive keratectomy-PRK) which can be achieved by selectively ablating corneal tissue from the anterior surface of the cornea. This procedure is also predicated upon the characteristics of the cornea wherein refractive correction of vision deficiencies can be achieved by varying the front surface curvature of the cornea. These methods are effectively based on the fact that the anterior surface of the cornea provides 80% of the total refractive power of the human vision system, and the rest is provided by the posterior surface of the cornea (negative lens) and the internal crystalline lens. Accordingly, relatively small changes in corneal curvature can significantly affect the focusing ability of the eye. Because of the manner in which shaping has been produced to date, this method has been used only in flattening out the surface of the cornea (decrease of curvature) by selective removal of the corneal tissue closer to the beam axis, i.e., suitable only for correction of low or medium myopia and mild astigmatism. Increase of curvature, for correction of hyperopia, with peripheral ablation is possible but is more difficult, and is not yet approved by the FDA.
The cornea comprises a thin protective epithelial layer on top of the Bowman's membrane or layer, which in turn covers the corneal stroma which is the thickest layer. While the epithelium is regenerative, the Bowman's membrane and stroma are not. With ablative corneal tissue removal procedures such as PRK, the epithelium and Bowman's membrane are removed together with a portion of the stroma. Subsequently, the epithelium regenerates on the exposed outer surface of the cornea but directly on the stroma, since the Bowman's layer is not regenerated. Direct regrowth of the epithelium on the stroma can however cause an undesirable corneal haze which gradually dissipates over time. Haze is also produced by the healing of the endpoint stromal surface layers which are badly mutilated by the ablation process. The resulting corneal curvature increases with time, i.e., regresses unpredictably.
Both RK and PRK, because of inherent instabilities and error factors, are also usually not suitable for correction of myopia of more than -6 diopters and PRK is not currently approved for corrections other than myopia.
A third surgical procedure known medically as Keratomileusis in situ (KIS) and also as Refractive Lamellar Keratoplasty (RLK) preserves the epithelium and Bowman membrane and has been used for corrections of up to -20 diopters. In such procedure there is an initial surgical removal, with a micro-keratome, of a uniform thickness button or lenticule of corneal tissue of a thickness containing the epithelium layer (intact), Bowman's membrane (intact) and a portion of the stroma. The button or lenticule preferably remains hingedly attached at one point to the cornea. The lenticule is moved out of the way, the stroma bed is then surgically reshaped, as required, and the lenticule is replaced, usually with adequate adherence and healing of the stroma-stroma surfaces and with the epithelium and Bowman's membrane being preserved, leaving the cornea clear. It appears that the stroma-stroma healing of lenticule-stromal bed interfaces of the RLK procedure reduces wound healing instabilities, making this procedure the most suitable for large refractive corrections. It also minimizes haze.
However, despite the advantage of retention of visual acuity and healing stability, the procedure is not very favored since it is complex, requiring high intra-ocular pressure, is expensive, is usually inaccurate, with high dependency on the surgeon's skill, and it can cause irregular astigmatism. These factors can be attributed to the high sectility and relatively generally unsupported character of a cornea, which makes use of a scalpel, or even a conventional micro-keratome, difficult and highly subject to inaccuracies and irregularities.
In a procedure described in co-pending application Ser. No. 08/304,245, filed Sep. 12, 1994, a device is described for use in shaped removal procedures with or without a flap with greater ease and accuracy in effecting corneal vision corrections. The device, in one embodiment, comprises a shaped template member as a deformation means (with the template being adapted in shape for specific corneas and desired final shape), wherein the template is placed and centered on the anterior portion of the corneal tissue to be removed, whereby it comprises a shaped surface therein to which the anterior portion, to be removed, is adapted to be fitted and deformed by such fitting.
The deformation is predeterminately controlled, such that the surface to be cut, at the base of this anterior portion, assumes a planar configuration, which is accessible for the cutting thereof. The shaped surface of the template has a height relative to a plane at the base of the template equal to the computed difference, point by point, of the difference in height between the anterior and posterior surfaces of the portion of the corneal tissue which is to be removed. The computed difference takes into account geometrical distortion and tissue compression/or extension. As a result, the posterior surface of the lenticule to be removed (i.e. the surface to be cut) assumes the planar configuration.
For different surgical requirements of refractive corrections, a series or catalog (standard) set of templates of appropriately differing shape and dimensions may be used, though specifically adapted custom templates based on the topographic mapping, may be readily constructed, and are preferred.
The shape of the template for a given desired correction depends on the relative position of the cutting plane and it is necessary that these portions be well established. Typical circular template dimensions are 6 mm in diameter, with deviations of the surface from planarity of 150 microns or less conforming to normal corneal corrections or irregularities. Preferably the surface of the template which comes in contact with the cornea is micro-roughened to prevent corneal slippage and lateral movement during subsequent cutting and to enhance suction.
The cutting means, is described as being a round, high speed rectilinear water (or sterile saline solution) jet produced by a water pressure of between 3000 to about 20000 psi and typically between 15000 to 20000 psi. It has been shown that a small diameter water jet beam of this character provides a very smooth transverse cut in corneal tissue, with a smoothness and integrity similar to that of the original tissue surface. In addition, since the cut is transverse, with little or no force vector directly into the cornea, no hydration of the cornea is detected with this procedure. The diameter of the jet is typically 30 .mu.m but even 75 .mu.m jets are suitable. The scanning speed is 5-40 millimeters per second and the cut occurs in one second or less. It has been demonstrated that no blade produces a cut that is less damaging to the tissue and generally it is much more damaging. The total water usage is about one drop.
Corneas are never perfectly spherical and it is important that the deviations therefrom be accurately, topographically pre-determined for effecting appropriate refractive and therapeutic corrections, in any of the aforementioned procedures. With the modified lamellar keratoplasty procedure the determination is also necessary in forming the template.
In order to effect the requisite topographical measurements, and in view of the only 3% reflectivity of a typical cornea, it has been the practice to use various ultrasonic and modified optical methods in mapping the irregular topography of the anterior of the cornea, as a guide for effecting the reshaping of the cornea. The common methods of topographical mapping, such as use of ultrasonic-ranging even when computer aided (as disclosed in U.S. Pat. No. 4,721,379) have however generally entailed problems in providing the very accurate mapping required (changes on the micron level are required for effective reshaping and ultrasonic mapping does not always provide such degree of accuracy). Other methods, such as disclosed in U.S. Pat. No. 5,116,115, involve the more accurate optical reflective mapping, but, because of the inherent transparency of the cornea, the method disclosed therein requires use of invasive reflective cover materials which must be placed on and exactly conformed to the anterior surface of the cornea to provide reflectivity, to provide a modicum degree of accuracy. In these techniques the number of data points, or resolution, is substantially limited.
At present, in view of deficiencies in very accurate individual topographic mapping, as well as in inherent deficiencies in the corneal reshaping laser instrument, utilization of the excimer laser to provide refractive vision corrections has resulted in refraction corrections of myopia with an accuracy of only about .+-.1 diopter, far less than that obtainable with corrective external lenses.
Most of the corneal topography machines provide a map of local corneal curvature which directly relates to refractive power at the point where the curvature is determined. For accurate reshaping of the cornea it is necessary to have an elevation map. The curvature map can be derived from the elevation map but the reverse is not true. The basis for this is imbedded in the mathematics of curve fitting.
It is therefore an object of the present invention to provide an extremely accurate non-invasive reflective topographic mapping method and device, suitable for use with a nearly transparent cornea, in corneal reshaping.
It is yet another object of the present invention to provide such topographic mapping device and method for use in shaping a corneal template for reshaping the cornea by means of a keratome such as formed from a water jet.
These and other objects, features and advantages of the present invention will become more evident from the following discussion and drawings in which: